Electrostatic printing provides a method of producing high resolution images based on the electrostatic attraction of toner particles to an electrically charged image. Images can be created either electrophotograghically, as in a xeroxgraphic copier, or digitally as in a laser printer.
In the electrostatic printing process a plate having a permanent or persistent image or pattern of insulating material on a grounded conductive backing is charged, e.g., by corona discharge, to produce a latent image of electrostatic charge on the insulating areas, the image is developed by toning with oppositely charged particles, and the toner image is then transferred by electrostatic or other means to another surface such as paper or polymeric film. Since the original image is permanent or persistent, multiple copies can be made with a single exposure by repeating the charging, toning and transfer steps. A general description of the well known electrostatic printing process, including a number of examples of methods for preparing the electrostatic "master" is disclosed in U.S. Pat. No. 4,732,831) issued to Riesenfeld, et al.
The electrostatic printing process would be highly desirable for printing complex, high density, fine line resolution circuit traces. Exceptionally high resolution can be achieved with liquid toners which are a suspension of charged particles in a dielectric liquid medium. However, Conventional liquid toners use electrically insulating resin particles to form the image, and in many cases other polymeric organic binders for adhesion of the image. The images formed from these toners do not have highly conductive electrical properties. These liquid toners are generally used for the printing of high resolution black and white or color images. The printed images are organic in nature and therefore not electrically conductive.
U.S. Pat. No. 5,011,758 issued to Detig, et al., proposes using a modified electrostatic printing process in the manufacture of printed circuits. Detig et al., replaced a number of steps of the well known process of 1) photoimaging of a film resist on a circuit board substrate, 2) removal of unexposed film resist, 3) etching away the exposed conductive substrate, and 4) removing the remaining (developed) film resist to produce a circuit trace pattern. Detig et al., employ an electrostatic printing process using a "film resist" toner (liquid electrographic toner) to deposit the circuit pattern of resist on a conductive circuit board from a "master" pattern image, thus by-passing the photoexposure and resist development steps. The Detig et al., process still requires the use of highly corrosive and highly toxic chemicals for the etching and stripping steps.
The present invention overcomes the limitations of the known art and uses a modified electrostatic printing process with a new liquid metallic toner to print images on printed circuit boards and other electronic devices which can be cured to electrically conductive metal traces by heating. Unexpectedly, it has been found possible to electrostatically print a metallic toner on a photographically imaged photoresist and thermally cure it to a well-bonded, well-consolidated metallic conductor.